1. Field of the Invention
This invention relates to a video tape recorder capable of recording and reproducing teletext signals together with video signals.
2. Description of Related Art
It is known to broadcast teletext data multiplexed with a broadcast television signal. The teletext data may represent characters for providing information such as news, weather forecasts, traffic information, quiz programs, characters to be superimposed on the broadcast video picture and so forth. Moreover, it has been known since the end of 1985 to broadcast teletext data multiplexed with a broadcast television signal in which the teletext data represents graphic patterns and/or additional audio information as well as characters. The teletext data also may include codes representing bit-mapped graphics.
Teletext broadcasting systems are in operation in North America (the NABTS system), England and France, but there are minor differences among the respective systems. A teletext broadcasting system is also in operation in Japan, but the Japanese system differs from the systems just mentioned in that the Japanese system is capable of broadcasting teletext codes representing sound and Japanese characters, while the other systems are not. The term "teletext" originally arose with respect to the North American, English and French systems, which are all oriented toward transmitting text in a 26-character alphabet, but all of the teletext systems referred to above are fundamentally the same, and will therefore be collectively referred to herein as "teletext broadcasting" systems.
In order to prevent the teletext broadcast signal from disturbing the video and audio signal included in the television broadcast signal, the teletext broadcast signal is multiplexed in the vertical blanking period of the television broadcast signal. At the current time in the Japanese system, the teletext broadcast signal is included in 8 lines of each frame, namely lines 14, 15, 16, 21, 277, 278, 279 and 284. According to an existing standard, it is contemplated that up to 16 lines (lines 10-16, 21, 273-279 and 284) may be used.
According to standards for teletext broadcasting in the European countries, up to 32 lines may be used for the teletext broadcast signal (lines 7-22 and 320-35), but in practice, each broadcasting station uses different ones of these lines for the teletext broadcast signal, while using others of these lines for a VPS signal, a test signal or the like.
FIG. 16 illustrates a format in which teletext broadcast information is provided in a video line according to the Japanese teletext broadcasting system. As shown in FIG. 16, the teletext signal consists of 296 binary bits per line, with "0" values represented as the pedestal level, and "1" values represented at 70% of the white level. The digital signal in each line includes a synchronizing portion and a data portion. The synchronizing portion includes a bit synchronizing code (also known as a Clock Run-In or "CR") which is provided for bit synchronization, and a byte synchronization code (also known as a Framing Code or "FC") which is provided for byte synchronization.
The data portion includes a prefix ("PFX"), information data and a check code. The prefix is composed of an 8-bit service identifying code ("SI/IN") which indicates whether the data is provided under a bit-mapped or "pattern" system, and a 6-bit packet control ("PC") which represents control information regarding transmission of the data portion. The prefix is followed by 176 bits (i.e., 22 bytes) of information data and 82 bits are added as a check code. The North American, English and French teletext systems are conceptually the same as the Japanese teletext system just described but differ from the Japanese system in some details.
A single teletext program requires approximately several kilobytes of data, depending on program content. Accordingly, since at most 176 bits are transmitted per line and dozens of programs are transmitted via one channel, each teletext program is transmitted at an interval of about 20 to 30 seconds.
In currently available teletext broadcasting systems, the teletext information can be viewed by means of a teletext broadcast tuner or a television receiver with teletext reception capability, either of which decodes the teletext data into displayable form. Although the decoded (i.e., displayable) teletext information can be recorded on a video tape recorder (VTR) or the like, raw teletext data that has not yet been decoded cannot be recorded by a conventional analog VTR because of the high clock rate (5.7 Mhz to 6.9 Mhz) of the teletext data within the television broadcast signal.
As indicated above, teletext broadcast information cannot be recorded in the form in which the information is broadcast by existing home-use analog VTRs or similar devices. On the other hand, if it is desired to decode and then record the teletext information, devices such as a teletext decoder IC, a large capacity memory, a Kanji ROM (read only memory), a control microprocessor and the like must be included in the VTR. However, and particularly in the case of a home-use VTR, the resulting increase in cost and the space required for the additional components are significant.
Even if the cost of such a home-use VTR can be reduced by mass-production or the like, there is an additional difficulty in that the VTR must be placed in standby mode for approximately 20 to 30 seconds while recording one teletext program. Accordingly, during the period of time in which the teletext information is recorded, the VTR must be kept in a stop mode or previously received information must be recorded during that time. Further, the above-mentioned typical 20 to 30 second teletext program period is only an average figure, and is subject to substantial fluctuation, varying from about 2 seconds in some cases to upwards of 30 seconds in other cases.
Moreover, there is a complete lack of synchronization between the standard recording rate for video and audio, which is 50 Hz or 60 Hz, and the recording rate for a decoded teletext program, which is once per 20 to 30 seconds. As a result, it is not possible to establish a correlation between the two recording rates.
As a result of the foregoing circumstances, if it is contemplated to record video, audio and teletext information in respective areas on one recording track, either video and audio on one hand, or teletext on the other hand must be given priority. However, if video and audio recording is given priority, then it is possible that the teletext recording area will be left blank, whereas if recording of teletext is emphasized, it is possible that the video and audio areas will be left blank. For this reason, the conventional method of recording teletext information includes decoding the teletext information and recording the same in the form of video and audio information. However, this approach suffers from the drawback that the accompanying television program broadcast simultaneously with the teletext information cannot be recorded.
Just as audio signal recording has evolved from analog recording on magnetic tape cassettes to digital audio tape recording, it is also now being proposed that analog video tape recording formats, such as 8 mm and VHS formats, are to be succeeded by digital video tape recording. It is contemplated by the present invention to take advantage of the 13.5 Mhz fundamental sampling rate used in digital video tape recording, which can conveniently accommodate the clock rate used for multiplexed teletext information broadcasting. It is further contemplated to record video, audio and undecoded teletext data together in a single track. Such an approach would avoid the aforementioned disadvantages encountered in attempting to record teletext information by analog video tape recorders.
By recording the teletext data without decoding the same, i.e., in the same form in which it is broadcast, increases in the manufacturing cost of the recorder, and the extent of the tape track area used for recording the teletext data, can be reduced in comparison to a system in which decoded teletext data is recorded. Further, because of the relatively small tape track area to be occupied by the non-decoded teletext data, it will become commercially feasible to provide product offerings in which teletext data is decoded by a teletext broadcasting television receiver.
An important advantage of digital video tape recording is that the same 13.5 Mhz sampling frequency can be used for both the 60 Hz and 50 Hz television signal formats. Accordingly, and unlike conventional analog VTRs, the same rotation rate of the rotary magnetic head drum can be used regardless of whether the digital video tape recorder is to be used for recording in the 60 Hz or the 50 Hz format. Although digital recording of the 60 Hz format requires 10 tracks per frame and recording in the 50 Hz system requires 12 tracks per frame, so that different tape lengths are used per frame, nevertheless other parameters, such as track pitch, track width and so on can be made the same for digital recording of both the 60 Hz and the 50 Hz formats.
It will be understood that the television tuning circuitry to be provided in a digital VTR varies depending on the country in which the DVTR is to be used, and the technique for handling teletext recording also must vary. It will also be appreciated that mutually different tape recording formats result from recording signals in the 60 Hz and the 50 Hz formats. However, tapes recorded in the 60 Hz format in one country, such as the U.S., can be reproduced in another country such as Japan, that also uses the 60 Hz format. Accordingly, it is desirable to provide only two different teletext recording formats, respectively corresponding to the 60 Hz and 50 Hz television signal formats. However, it is to be noted that there are presently four different teletext broadcasting formats, respectively for Japan, North America (NABTS) the U.K. and France, having parameters as shown on table 1 below.
TABLE 1 ______________________________________ North America ITEM Japan (NABTS) U.K. France ______________________________________ clock 364 f.sub.H 364 f.sub.H 444 f.sub.H 397 f.sub.H frequency full 296 bits 288 bits 360 bits 320 bits length of data line clock run- 16 bits 16 bits 16 bits 16 bits in framing 8 bits 8 bits 8 bits 8 bits code (length) prefix 14 bits 40 bits 16 bits 16 to 40 bits data block 176 bits 224 bits 320 bits 240 to 280 bits suffix 82 bits 16 bits 0 0 framing 11100101 11100111 11100100 11100111 code (E5h) (E7H) (E4h) (E7h) (value) ______________________________________
It is also desirable that a teletext recording system be sufficiently flexible to accomodate future expansion of teletext data broadcasting using more than the 8 lines per frame currently used in Japan, for example.
Another consideration with respect to recording teletext information on tape, and reproducing the information from the tape, is dealing with "drop-outs" to which tape is inherently subject. Even though some error correction capability can be provided by way of a check code or the like, it is also necessary to pay special attention to the possibility of burst errors, such as may be caused by a drop-out or the like. When the teletext information represents alphabet characters, the risk of data loss is not great, but when Japanese characters such as ideographs are represented by the data, even the loss of a single word of the teletext data may make it impossible to recover a Japanese character.
Furthermore, since home-use digital VTRs will undoubtedly use data compression techniques in order to conserve the required length of tape, it should be anticipated that compression technology will continue to advance, so that, for example, a frame of data that can currently be recorded using 10 tracks may in the future be recorded using only 5 tracks. Therefore, it is desirable for a teletext data recording format to be sufficiently flexible to accommodate future improvements in compression technology.